Download UP AND DOWN THE AIR

UP AND DOWN THE AIR
Video: DVD, 13 minutes, 2005
Addressees: pupils from 10 years of age at elementary,
secondary and grammar school
Keywords: windward and leeward, the High Tatras, air
moisture, point of condensation, summer clouds, storm, area of
high and low pressure, cold front, adiabatic warming and
cooling, cloud formation, Föhn principle, Dobšinská ice cave
Chapters: Windward and Leeward, Summer Clouds, Storm,
Ice Cave
A Film by Rainer Hahn
Assistance: Jana Bryjová, Annerose Hahn, Jakub Ľudma,
Rastislav Pjontek, Miroslava Suchánová, Katarína Frečková
Solution suggestions to the
worksheet:
Narrator: Katarína Frečková
1a. emergence of water vapour
droplets during cooling
1b. fog, clouds and precipitation
1c. temperature, at which water
vapour condenses
1d. the side exposed to/ away from the
wind
1e. warm, moist air rises on the
windward side, condenses, clouds
emerge, on the lee side the clouds
dissipate due to adiabatic warming
Content:
The film shows consequences of adiabatic warming and
cooling of the air: emergence and dissipation of clouds;
condensation niveau of the clouds on windward and leeward
side of the mountains; the föhn principle and its consequences.
It also illustrates the formation of summer clouds and storms,
as well as shifting of cold front and formation of an ice cave.
In three natural landscapes – high mountains, flatland and a
cave – the film provides an impressive insight into enigmatic
powers of the air. Up and down the air – that is a constant
circulation.
2a. small, cumulus clouds
2b. the air cools down
2c. e.g. condensation trail does not
dissipate
2d. cold and warm air masses collide,
the cold front takes over warm front
2e. energy
3a. cold air in winter cools the cave,
warm air does not get in
3b. ground glaciers and various forms
of ice all year round
4a. snow, rain, hoarfrost, hail, black
ice, frost, etc.
4b. cloud formation on windward side,
cloud dissipation on the lee side
4c. North of Slovakia, at the Polish
border
Praxis Unterrichtsfilm
Draisendorf 1
91346 Wiesenttal
1
UP AND DOWN THE AIR
Film text to PRAXIS EDUCATIONAL FILM
Windward and Leeward
Ahead of us: the High Tatras, the smallest high
mountains in the world. The High Tatras are
a famous holiday and skiing resort, as well as
the location of spas for curing pulmonary
diseases. The peaks are thickly veiled by the
clouds. Such a common view! The airflow
approaching the High Tatras blows from the
north to the south.
This is the Polish side. The town of Zakopane, a
noted winter sports resort, is typical of almost
constant stormy and rainy weather. That is the
Slovak side: the city of Poprad suffused with
sunshine. Between the two cities, on an area of
about six kilometres (approx. four miles),
sprawls the mountain range of High Tatras.
Why is the weather on such a small area so
unstable? That’s what we’d like to find out! So,
let’s go to the Lomnický Peak.
On the funicular railway, we go over beautiful,
clear, but ice cold brooks to reach the belt of
dwarf mountain pine. We are approaching the
peak. Thick clouds embrace us. The air is humid
and it is getting colder and colder. The peak
offers a breathtaking, bright view southwards.
Suddenly the curtain of clouds opens. It gives us
a chance to observe the formation of clouds.
Warm air, rising from the valley, is humid. On its
way up, it cools down and the moisture
condenses into very small water droplets. The
clouds arise. The point of condensation is
clearly visible. Mountain peaks are heavily
overcast. On the other side, we can observe
quite a reverse process. The clouds sink down
the peak and dissipate. This is the point of
dissipation. The cool air heats up.
The High Tatras – a cloud machine.
2
Summer clouds
In the flatlands the air circulation is similar to
that in the mountains. The sun heats the
earth and the warm air rises up. The higher
the air rises the lower is the temperature.
The warm air cools down and the water
vapour condenses. Summer clouds arise.
Most of the summer clouds are visible
between one and two p.m.
The darker the bottom of the cloud the
higher it reaches.
In the afternoon, the air gets colder again
and clouds disappear.
On the next day, the summer clouds are
formed again. However, this time they
become larger and more numerous.
Eventually, there comes a short, strong
summer storm. Soon, everything is calm
again and the air is fresh and clear.
UP AND DOWN THE AIR
Film text PRAXIS EDUCATIONAL FILM (continuation)
Storm
How does a storm originate?
It is a lovely day. This is an area of a high air
pressure with warm air. Cirrus clouds fly high
above us. A condensed stripe left by an airplane
does not dissipate, because the air moisture
strongly increased. We can see the low air
pressure area approaching.
The next day begins with a nice sunny weather.
When a low-pressure area with colder and
moister air comes, the colder air will move over
the warm air of the high-pressure area. The
warm air has no more space to rise and the
pressure between the two layers steadily
increases.
Electric discharges occur. A strong storm
develops.
These are the consequences of the storm: A
new house was hit by a thunderbolt. Its statics
was badly damaged. It must be demolished.
The storm goes on.
Ice Cave
This is a unique ice cave in Slovakia – the
Dobšinská Ice Cave. One of three such
caves in Europe.
It is August and swarms of tourists visit this
natural attraction. The huge hole is
fascinating by its astonishing ice formations,
stalactites, ice tunnels and ice waterfalls.
It is high summer. The appearance of the
cave constantly changes with the alternation
of the thawing and freezing periods. Old
photographs show a huge ice waterfall. It
looks quite different today. When the cave
was discovered, it was used as an ice rink.
On some places, the ice can be up to 26.5
metres (27 yards) thick!
How can the cold air and ice remain here even in
summer? On the highest spot, the ice cave has got an opening – a gap in the rock. Through this
aperture, the cold air in winter flows into the cave as if through a funnel. Due to a special location
of the cave, only the air, which is colder than the temperature inside, can get in. In winter, the cave
becomes so chilled, that the glacier and other ice formations can last even through the whole year.
3
UP AND DOWN THE AIR
Supplementary material to PRAXIS EDUCATIONAL FILM
Condensation
Saturation of the air with water vapour is a basic condition for condensation and cloud formation.
Water vapour is an invisible gas. What we can see during heating radiation from thermal power
machines or during exhaling in cold air, are products containing – besides the actual water
vapour – also products of condensation in the form of very small droplets of water. Water vapour
or water droplets enter the atmosphere in the form of fog, clouds and precipitation.
For condensation, the air must be cooled down to the dew point; it has to be supersaturated with
water vapour.
Condensation is caused by the following processes:
1. Cooling down through heat radiation: The surface of the earth loses energy, especially during
cold nights, by means of heat radiation. When the radiation reaches the dew point temperature,
dew arises. When the dew point temperature is reached near the ground, ground fog arises.
The radiation area can also be on the obverse side of the clouds. The cloud cools down and the
resulting cold air provides for condensation and increase in cloud volume.
2. Cooling down through heat exchange: Due to the approach of warm air (advection), the air
above cold water, snow or cold continental regions cools down up to the dew point temperature.
Fog builds up. When the cold air streams over warm water areas, such as seas or shores of the
subpolar regions, a temperature exchange occurs between the water and fog due to continental
winds. Such fog is often called sea fog.
3. Cooling down as a result of blending of warm and cold air: Approximation of warm and cold air
masses leads to creation of an interface between air masses. In this way, the so-called front
arises. On such a front, cooling of the warm air ensues, with condensation and cloud formation.
4. Cooling due to adiabatic expansion of the air (convection): During the expansion, the
temperature drops by one degree per 100 metres. The cooling process up to the dew point
temperature triggers condensation and cloud formation.
4
UP AND DOWN THE AIR
Supplementary material to PRAXIS EDUCATIONAL FILM (continuation)
Fog
Industrial areas and conurbations with heavy emissions of condensation nuclei and water vapour
show high frequency of fog. Also, valleys and lowlands are more liable to the occurrence of fog.
It can be often observed in coastal and boundary regions on the interface of warm and cold air.
Fog is dissolved by heating or exchange of air masses. Rain and snowfall may dissipate fog,
similarly as it can be dispersed by trees.
Clouds
Fog turns into clouds in higher layers of troposphere. They are a suspension of water droplets or
ice particles. Clouds can be defined horizontally and vertically and they can move in both
directions. Vertical shifts cause the existence of stratus clouds, convective processes result in
cumulus clouds. Besides, there are water clouds, ice clouds and mixed clouds. Register of
cloud types lists four cloud families with altogether ten cloud types. Moreover, there is a series of
subtypes. Clouds cannot be unambiguously recognized from satellite pictures. In the same
situation, there is a large scale of existing shapes.
Large-scale convergence, convection, thermal effects of the surface of the earth and orographic
influence result regionally in the cycle of seasons and, according to the properties of the present
air masses, in various cloud conditions and cloud forms.
Cloud Formation
Saturation of the air with water vapour is the basic condition for cloud formation. Value of air
saturation is dependent on temperature. The warmer the air becomes, the more water vapour it
can absorb. Cooling of the air by any kind of air moisture results in such a temperature, at which
the air is saturated with moisture. This value is called the dew point temperature. When air
reaches this point, the moisture is condensed. This leads to cloud formation or precipitation.
Thus, the air moisture is dependent on temperature.
Clouds arise: A. above warm air areas: the warm air rises, cools down and condenses; B. when
warm air approaches cold air areas; C. on the fronts; and D. on geographical elevations or
mountain passes
5
UP AND DOWN THE AIR
Supplementary material to PRAXIS EDUCATIONAL FILM (continuation)
Föhn principle as a thermodynamic phenomenon is apparent on the wind and lee side of a
geographical elevation. Föhn principle is an atmospheric process, taking place in an air mass
when overcoming an orographic obstacle.
In the föhn principle, the air pressure and temperature decrease with height, while the proportion
of pressure and temperature remains constant up to the point of condensation. Cooling of the air
results. Such a change of state without an increase or decrease in heat is called adiabatic.
The air can only absorb such an amount of moisture, which corresponds to the temperature.
During an upward motion of an air volume the temperature increases by 1° per 100 metres. The
moisture (the share of water vapour in air volume) increases up to the dew point. The air is
100 % saturated. If the air rises further, water vapour condenses.
As the hilltops or mountain passes are located in quite high altitudes, drizzle or rain often occurs
during condensation and therefore the descending air on the lee side displays slight air moisture.
Now, the decrease in temperature per 100 metres is even more minute. On the other side of the
obstacle, the air mass is heating up under rising pressure and the remaining clouds reach the
point of dissolution.
Now the air can absorb more moisture and thus evaporate condensation droplets. The air
becomes warmer. By different altitudes of the windward and leeward sides are the temperatures
adequately different. Thus, the mountain functions as a sieve for the moisture and also heats the
air.
The air comes on the lee side warmer, dryer, with a high saturation deficit and leads to high
evaporation in the landing area. Several days of clear weather follow, with unusually far range
of vision.
The result of adiabatic heating and cooling are the origin and dissolution of clouds.
Condensation and dissipation niveau of the clouds is changed according to the volume of
moisture, temperature and elevation.
6
UP AND DOWN THE AIR
Supplementary material to PRAXIS EDUCATIONAL FILM (continuation)
Precipitation
A specific form of precipitation is storm.
Condensation yields water droplets. In
order to release precipitation, bigger water
drops have to emerge first. However, it
was discovered, that ice crystals arise in
the clouds to fulfil certain conditions.
During precipitation, the ice-crystals fall
through warmer layers and reach the earth
as drops. Otherwise, it would not be
possible, that ice-crystals in winter arise
from water droplets in the bottom layers;
thus, these must have emerged first.
Arguments supporting this theory are not
yet sufficient, though.
Basic conditions for a storm are large scale
condensation of water vapour and convective
changes of state. During the discharge of
condensation heat, high temperatures and moist
air prefer strong vertical currents. Therefore, the
storm clouds are mostly of high towering shapes.
A storm normally consists of several layers of
clouds, which collide with each other. By means of
the following condensation between cold and
warm air, or through changes conditioned by
temperature and height, great amount of heat and
other energy is released into the atmosphere.
These are discharged as lightning and thunder.
Approximately 44 000 storms and about 8 million
cases of lightning are discharged daily all across
the world. Most of the lightning flashes discharge
in the upward direction, only a few are headed
towards the surface of the earth.
There are several forms of precipitation,
e.g. rain, snow, hoarfrost, hail, black ice,
frost, etc.
A storm of average strength has the power of
several hundred megawatts, which equals the
capacity of a small nuclear power station.
Classification of storms distinguishes three types
of storms:
1. Air mass storms are heat storms and
emerge due to overheating of the layers
close to the ground, when the air is highly
moistened. This is typical of tropes and
summer storms;
2. front storms occur mostly along cold fronts,
when warm air moves over the cold air.
Such storms can take a long time and they
occur most frequently in Europe/central
Europe, where cold continental polar air
overtakes the warm air of the Golf stream.
This can also happen conversely, when
cold air takes over warm air, which cannot
rise any higher to cool down. Resulting
pressure between the air masses is
discharged;
3. Orographic storms occur when advective
air glides over mountains, with unstable
humidity and swells into great heights.
These stormsx are often long lasting.
7
UP AND DOWN THE AIR
WORKSHEET to PRAXIS EDUCATIONAL FILM
1a. How would you explain the notion
of condensation?
1b. In what forms does water vapour
occur in the atmosphere?
1c. What does dew point temperature
mean?
1d. Can you explain the terms
windward and leeward?
1e. Can you explain the emergence
and dissipation of clouds on the
example of föhn principle?
2a. Can you describe summer clouds?
2b. Why do they dissipate in the
afternoon?
2c. What are the signals forecasting a
rainy weather?
2d. What fronts collide during a storm?
What happens then?
2e. What is discharged during thunder
and lightning?
3a. Why does the cold air and ice hold
in the ice cave?
3b. What are the consequences of
strong cooling of the cave in winter?
4a. What forms of precipitation can you
name?
4b. Why can we call the High Tatras a
cloud machine?
4c. Describe the location of the High
Tatras with the help of a map.
8